1. Field of the Invention
The present invention relates to electrostatographic development apparatus, and more particularly to such a development apparatus that includes a magnetic development brush which has an extended development nip.
2. Description Relative to the Prior Art
Development apparatus which include magnetic development brushes are well known for use in electrostatographic reproduction apparatus, such as copiers or printers. Such development apparatus which are used for developing latent images formed electrostatically on image-bearing surfaces in reproduction apparatus are disclosed, for example, in Japanese Patent No. Sho 55 (1980)-39406; U.S. Pat. No. 4,235,549 issued to Eisbein et al; 4,287,850 issued to Yamamoto et al.; and 4,928,145 issued to Okamoto et al.
As shown, the magnetic development brush of each of the disclosed development apparatus includes a generally cylindrical magnetic core and a generally cylindrical non-magnetic shell surrounding such core. When mounted for operation in a copier or printer, for example, the non-magnetic shell of the magnetic brush is located spaced from, and forms a development nip or footprint with the image-bearing surface of such a copier or printer. As further shown, the magnetic core of the magnetic brush functions to attract magnetic developer material contained within the development apparatus onto the surface of the non-magnetic shell thereof, thereby forming a developer material nap on such shell. Thereafter, the rotation of the magnetic core then causes such a developer material nap to move or be transported over the surface of the non-magnetic shell into and through the development nip where the developer material nap contacts and develops electrostatic latent images on the image-bearing surface.
The quality of the image so developed depends in significant part, for example, (a) on the nature, and height or thickness of the developer material nap being moved through the development nip; and (b) on the length of development time during which the latent images on the image-bearing surface are in desired contact with such a developer material nap. Such development time, at any given development apparatus speed, is determined of course by the size of the width or footprint of the development nip. As disclosed for example in the '549 patent to Eisbein and in the '850 patent to Yamamoto, such image development quality can be improved in part by providing means such as an extended shoulder section of the non-magnetic shell for increasing the width of the development nip, that is, for increasing the size of the nip width or footprint of the development nip, and thereby increasing the development time.
Unfortunately however, such an extended shoulder section will lie in an area significantly remote from the magnetic core such that it will have a weaker magnetic field strength relative to the rest of the areas of the non-magnetic shell. When such an extended development nip is formed in a generally horizontal orientation with the image-bearing surface, for example in a 6 o'clock position with a cylindrical image-bearing surface, the magnetic developer material nap must then move substantially vertically and against gravity in order to reach the extended shoulder section for example on an entrance side of such a nip. The consistency and desired thickness of the developer material nap has been found to be detrimentally affected during such substantially vertical movement. As a consequence, a consistent and desirably thick developer material nap, earlier formed at the base of the non-magnetic shell, may not efficiently reach and move around such an extended shoulder section in order to enter the extended development nip for quality image development.